This SBIR proposal intends to prepare luminescent anti-PSA (prostate specific antigen) single chain antibody fragment (scFv) -- Quantum Dot (QD) conjugates that can be used for PSA immunoassays. This project will be built on our recent success in preparing luminescent, monodisperse, super stable, water-soluble, and bio-compatible dendron-boxed semiconductor nanocrystals, as well as on this Pl's earlier works in the cloning, expression and structural characterization of an anti-PSA single chain antibody fragment-scFvB80. The success of this proposal could provide a new path of making antibody-QD conjugates. In comparison with current antibody-adaptor protein-QDs conjugates, this proposed scFvB80-QDs would be directly coupled and could have the advantages of small size, simple structure, easy preparation, low production costs, and ready modification via genetic engineering. The proposed anti-PSA scFvB80-QD conjugate could have the potential for improving the PSA immunoassay.